1. Field of the Invention
This invention relates to boreholes and/or producing wells and more particularly to steam-injection wells.
2. Description of the Prior Art
The injection of steam into a well may prove necessary in the working of a mineral deposit or in geothermal power development. In the first case, the injected steam heats the deposit in order to improve the oil/water viscosity ratio. In the second case, the injected steam permits heat storage for subsequent heat recovery.
In both cases, it is essential to ensure that the greater part of the injected steam is employed for the achievement of the desired objective. To this end, devices used for injection of steam must afford the highest possible degree of leak-tightness. However, taking into account the temperature of the injected steam, it often proves difficult to attain such a degree of leak-tightness. The difficulty arises from the fact that devices and materials which are capable of achieving perfect steam-tightness are not currently available. This difficulty is aggravated by the fact that the materials employed in steam-injection wells are subject to processes of expansion and/or conversion which take place in particular at the time of an increase in temperature, for example of the injected steam.
Steam is injected into a string of tubes employed as injection or production tubing placed within a so-called casing string which consists of outer tubes of larger diameter. Injection takes place at considerable depths and more precisely at substantially the same depth as the mineral deposit to be worked or of the reservoir layer of heat to be stored. In consequence it has always been proposed to place the sealing device as close as possible to and above the steam injection level. In the example of a mineral deposit borehole, a connecting and sealing unit designated in current practice by the term "packer" is placed near the bottom of the well bore between the tubing string and the casing string above the orifice or orifices formed in the cylindrical wall of the casing string through which the injected steam passes into the mineral deposit to be worked. Thermal expansion of materials which are liable to undergo deformation such as elongation as a result of a temperature rise is absorbed by a sliding seal placed opposite to the packer and between the packer and the casing string. Since the sliding motion of a seal of this type gives rise to leakages, the injected steam passes during said sliding motion through the small passages formed as a result of wear or aging and then returns upwards to the wellhead. In order to repair a defect of this type, it is necessary to make use of drilling equipment in order to retrieve the packer and tubing string, to change the sliding seal, to return the packer and tubing string assembly downwards into the well and to position said assembly directly within the casing. These different operations make it necessary to cool the well either naturally by allowing it to revert to the adequate temperature for permitting replacement of the sliding seal, which involves an appreciable loss of time, or to produce a thermal shock with all the consequences which this is liable to entail in the case of the other elements and/or devices placed within the well.